The present invention relates to apparatus and method for encoding and decoding digital video data and, more particularly, to apparatus and method for encoding and decoding digital video data which removes noise from subtitle data included therewith.
As is known, subtitle data which represent subtitles that are to be superimposed on a video picture are stored with the video signal prior to the recording or transmission thereof. In one system, known as the CAPTAIN system, subtitles are transmitted as a character code of dot patterns. In the known data format CD-G (compact disk graphics), graphic images as well as subtitle data are stored in the subcode region of the data format. As shown in FIGS. 1(a) to 1(c), which schematically illustrate the data structure of the CD-G format, one block of data is comprised of 98 frames which are each comprised of a one byte sub code area followed by 32 bytes of data. The 32 bytes of data in one frame are comprised of 6 four-byte samples and 8 bytes of error correction code, as shown in FIG. 1(A), and each sample is comprised of two bytes of L channel data and two bytes of R channel data. As shown in FIG. 1(b), one block is comprised of 98 frames, i.e., frames 0 to 97, and each block also is divided into 8 channels P, Q, R, S, T, U, V and W, such as shown in FIG. 1(c). The subcode data of frames 0 and 1 are stored as sync patters S0 and S1 in each of the 8 channels P to W, and the sync patterns in channels R to W each are followed by a 96 byte data area in which data of frames 2-97 are stored. Channels P and Q, following sync patterns S0 and S1, generally include "search" data which are utilized for searching tracks, and graphics data generally is stored after sync patterns S0 and S1 in channels R to W. Thus, 96 bits per channel.times.6 channels R to W=576 bits of graphics data are stored in each block. Further, when each block is transmitted at the cyclic frequency of 75 Hz, the data transmission rate for a block is 7.25 KHz (75 Hz.times.98 bytes), which results in the subcode transmission bit rate of 7.35 kbs (kilobytes per second).
FIG. 2 schematically illustrates the transmission format of channels R to W shown in FIG. 1, wherein a transmitted "symbol" is comprised of the combination of a single bit from each of the channels R to W, a transmitted "packet" is comprised of 96 symbols, and a transmitted "pack" is comprised of 24 symbols 0 to 23. The three bits of channels R, S and T of symbol 0 of each of the packs are reserved for "mode" data, and the 3 bits of channels U, V and W of symbol 0 of each of the packs are reserved for "item" data. Table 1, shown below, shows the various data modes as defined by the mode data and the item data.
TABLE 1 ______________________________________ Mode Item Data Mode ______________________________________ 000 000 0 mode 001 000 graphics mode 001 001 TV graphics mode 111 000 user mode ______________________________________
Instruction information is stored in symbol 1 of each of the packs and parity and other additional information are stored in symbols 2 to 7 of each pack. Graphics data generally are stored in symbols 8 to 19 of each pack, and error correction code are stored in the remaining symbols 20 to 23 of each pack.
In the above discussed format, each pack can store 6.times.12=72 bits of graphics data or 72 pixels of graphics data. If a single character is stored in symbols 8-19 (6 bits.times.12 symbols) of each pack, then 300 characters are transmitted per second at the bit transmission rate of 75 Hz. In the CD-G format, a single picture is comprised of 192 lines wherein each line includes 288 horizontal pixels and, thus, each picture is transmitted in 2.56 seconds, as shown in equation 1. EQU (288/6).times.(192/12)/300=2.56(sec) (1)
If each pixel value is expressed in hexadecimal form, and each hexadecimal representation of 1 pixel requires 4 bits, the amount of time required to transmit a single picture is approximately 10.24 seconds.
In the CAPTAIN System, 192 lines (248 horizontal pixels each) are displayed for each picture, but such a resolution is inadequate for the 720.times.480 resolution of digital television. In addition, subtitle characters stored in the above discussed data formats may become distorted (e.g., "jagged") due to aliasing or other phenomena. One technique to prevent subtitle characters from becoming distorted during the encoding or decoding thereof is to utilize high resolution filters to convert the digital images into "multivalued" data, but such filters generally are extremely expensive and they cause deterioration in the background images of the picture.
Typically, bit map data which represents the subtitle characters undergo a "thickening" process wherein the edge of the characters are "outlined", but such a process generally causes the characters to "smear" in the horizontal direction. Thus, the resultant quality of subtitles represented by bit map data generally is poor.